It is well known in the art that certain materials called thermoluminescent phosphors can be irradiated with high energy radiation and then subsequently stimulated using heat, to produce a luminescent emission. Thermoluminescent phosphors are in widespread use in radiation dosimeters used to measure the amount of incident radiation to which people, animals, plants and other things are exposed. Thermoluminescent dosimeters are widely used by workers in the nuclear industries to provide a constant monitor for measuring exposure to radiation.
Thermoluminescent phosphors are excited by energetic radiation such as ultraviolet, X-ray, gamma, and other forms of radiation. Such ionizing radiation causes electrons within the thermoluminescent material to become highly energized. The nature of thermoluminescent materials causes these high energy electrons to be trapped at relatively stable higher energy levels. The electrons stay at these higher energy levels until additional energy, usually in the form of heat, is supplied which releases the trapped electrons, thereby allowing them to fall back to a lower energy state. The return of the electrons to a lower energy state causes a release of energy primarily in the form of visible light which is ordinarily termed a luminescent emission.
The use of thermoluminescent phosphors in personal dosimeters has led to demand for a large number of dosimeters which must be read on a routine basis in order to monitor exposure of persons or other objects to ionizing radiation. Because of the substantial numbers and the relatively slow reading techniques currently employed, the job of reading dosimeters becomes very time consuming and costly.
There are four commonly known methods of heating thermoluminescent material in order to release the trapped electrons and provide the luminescent emission which is measured as an indication of the amount of ionizing radiation to which the dosimeter was exposed. The first and most common method for heating thermoluminescent phosphors is by contact heating. The second method is heating using a hot gas stream which is impinged upon the phosphor. The third method uses radiant energy in the form of infrared beams which heat the luminescent phosphor. The fourth method uses infrared laser beams to provide the necessary heat for luminescent emission.
Novel methods and apparatuses for laser reading of thermoluminescent phosphor dosimeters are disclosed in detail in applicant's co-pending patent application Ser. No. 652,829, now U.S. Pat. No. 4,638,163, the subject matter of which is incorporated by reference hereinto. One of the inventors of this invention and his colleagues have developed laser reading techniques and dosimeters, as disclosed in an article entitled "Laser Heating in Thermoluminescence Dosimetry," by J. Gasiot, P. Braunlich, and J. P. Fillard, Journal of Applied Physics, Vol. 53, No. 4, July 1982. In that article, the authors describe how thin layers of thermoluminescent phosphors can be precipitated onto glass microscope cover slides and used as laser readable dosimeters. Powder layers of the phosphors were in some cases coated with a thin film of high temperature polymers. The content of said article is hereby incorporated hereinto by reference.
Laser heating of thermoluminescent phosphors is superior because of the greatly decreased heating times and associated increased processing rates which are possible. Release of stored luminescent energy within a short period of time greatly improves signal-to-noise ratios and thus the accuracy of dosimeter measurements.
The benefits of laser heating for luminescent phosphors has not been fully realized because of difficulties associated with laser heating of prior art dosimeters. Relatively thick layers of luminescent phosphors (1 mm) can be heated using lasers, but necessarily require longer heating times because of the larger mass of phosphor which must be heated. Higher laser power levels can theoretically be used but at higher cost. Thick layer dosimeters further suffer from problems of prolonged heating as the heat generated in the phosphor diffuses outwardly from the area of last impingement. This reduces the accuracy of the measurements. Thermal gradients developed within the phosphor layer can also lead to degradation of the phosphor layer when high power levels are used to produce high heating rates.
Thermoluminescent phosphor radiation detectors have been commercially available as crystals, hot-pressed or extruded elements, powder embedded into a Teflon matrix, phosphor particles in glass capillaries, and in thin layers upon metal or plastic foil in surface concentrations of about 20 milligrams per square centimeter, E. Piesch, "Application of TLD to Personal Dosimetry," Applied Thermoluminescence Dosimetry, Editors M. Oberhofer and A. Scharmann, 1981, which is hereby incorporated hereinto by reference. The Piesch article also states that ultra-thin bonded discs of lithium fluoride (LiF) in a Teflon matrix have been bonded to thick Teflon bases. The article further states that such dosimeters use approximately 6 milligrams of phosphor per square centimeter. Such dosimeters are unfortunately also light sensitive and produce their own luminescent output. They are unable to withstand the approximately 400.degree. C. or higher temperatures which must be used to deep anneal dosimeters after many prior exposures and read cycles. Many other plastic and organic material based dosimeter configurations also suffer from these problems of temperature and light stimulated luminescent output. Plastic substrate and/or matrix dosimeters also are unacceptable for laser heating because of the localized high temperatures developed at the point of laser impingement which lead to degradation of the plastic. Other organic materials also suffer from these limitations.
U.S. Pat. No. 4,510,174 to Holzapfel et al discloses a method for manufacturing thin layer dosimeters. The method involves hot-pressing a thermoluminescent phosphor deposited on a substrate. The substrate must have a suitably prepared surface. The hot-pressing causes plastic flow of the phosphor to occur, bonding the phosphor to the substrate. The Holzapfel invention is disadvantageous in that it does not work well, if at all, with CaSO.sub.4, BeO, and Al.sub.2 O.sub.3 because these materials are hard and not subject to easy plastic flow at reasonable temperatures and pressures. The Holzapfel invention also is disadvantageous because the thickness of phosphor that must be heated is greater than what is required. The substrates used by Holzapfel are god for contact heating but are not good in laser heating because of their high conductivity. Thermal expansion rate mismatch is also a potential problem with laser heating of the Holzapfel dosimeters.
U.S. Pat. No. 3,894,238 to Cox et al teaches a laminated dosimetric card including thermoluminescent dosimeters sealed in an envelope of polyolefin, fluorinated ethylene propylene polymers, or PTFE. The dosimeters use a crystal or chip of phosphor or a quantity of powder. Such dosimeters cannot be deep annealed or heated using a laser reading apparatus, because of temperature limitations as explained above.
A manganese activated phosphate glass useful in radiation dosimetry is taught in U.S. Pat. No. 3,899,679 to Regulla. Regulla uses a phosphate glass which is doped with manganese in concentratios from 0.1% to in excess of 10%, along with dysprosium. Addition dopants disclosed by Regulla include cerium and silver. U.S. Pat. Nos. 3,294,700 to Bedier et al, and 3,463,664 to Yokota et al disclose other phosphate glasses useful as dosimeters. U.S. Pat. No. 3,255,120 to Cohen teaches a further thermoluminescent glass.
It is an object of this invention to provide thin layer thermoluminescent radiation dosimeters which can be very rapidly read using laser beams in a manner that allows reliable measurment of the resulting luminescent emissions.
It is another object of this invention to provide methods by which a laser readable thin layer of thermoluminescent radiation dosimeter can be fabricated.
These and other objects and advantages of this invention will be apparent from the description given herein.